Lieutenant Martin Foley of the Massachusetts state police displays one of various electronic devices planted at various spots in Boston. Photograph: Steven Senne/AP

A publicity stunt for a cartoon show threw security services in Boston into a panic yesterday because of fears the city faced a series of bomb explosions. In the biggest security scare in the city since 9/11, police and other units found 10 packages left on bridges and outside a medical centre, each with a blinking electronic device.

With the US more jittery than usual about a possible terrorist attack, news channels maintained regular updates throughout the afternoon as bomb squads went into action. Roads and bridges were closed off. The coastguard was called out to block off traffic on the Charles river and the subway was also suspended.

Speculation over who was responsible ranged from terrorists to, because of the medical centre, anti-abortionists, who often attack doctors and clinics.

Authorities said some of the objects looked like circuit boards or had wires hanging from them. One was found attached close to a bridge span. The bomb squad blew up one of the packages.

Concern turned to relief as the police reported no explosives had been found and then to anger as it was revealed it was a publicity stunt. Deval Patrick, the governor of Massachusetts, said: "It's a hoax - and it's not funny."

Thomas Menino, the mayor, said a stiff penalty would be sought for those responsible. "It's about keeping a city on edge. It's about public safety," he said.

Turner Broadcasting, a unit of Time Warner, apologised: "We regret that they were mistakenly thought to pose any danger. The 'packages' in question are magnetic lights that pose no danger."

They were part of a campaign in 10 cities for the adult cartoon show Aqua Teen Hunger Force. The show includes a talking milkshake (Master Shake), a box of chips (Frylock) and a meatball (Meatwad). Two of the characters, Ignignokt and Err, were pictured on at least one of the packages. A film version of the show is due to be released next month. As soon as Turner heard about the scare, it contacted the police to offer precise locations.

The marketing company responsible for the campaign, Interference Inc, had no immediate comment.